The present invention relates to thermocouple connectors.
Thermocouples are a common form of contacting electrical sensor. They consist of two dissimilar metal wires, joined at their ends to form an electrical circuit. One of the junctions is attached to or embedded in the surface of the component to be measured and is referred to as the sensing or “hot” junction. The other is a reference or “cold” junction. Typically the two wires are formed into a single cable. The circuit generates a voltage due to the diffusion of electrons along the temperature gradient (the Seebeck effect), and traditionally materials have been chosen to give a voltage output approximately proportional to the difference in temperature between the two junctions. The small voltage generated is measured at the cold junction end using a voltmeter and is converted to a temperature using a calibration for cable type of the thermocouple. The cold junction temperature is usually measured with a resistance thermometer which is generally integral with a signal conditioning unit.
Cables are commonly supplied with responses conforming to internationally agreed tables within given tolerance bands, for example class 1 K and N type cable will produce the standard thermal e.m.f. response in the range −40 to >1000° C. to within a tolerance of ±1.5° C. or ±0.4% of reading. Increased accuracy can result from more accurately characterising individual thermocouples or a batch of cable from a single production run at points along its length (“batch wire calibration”).
Voltages will also be produced at all junctions of dissimilar metals in the circuit wherever temperature differences arise. Thus, to minimise errors, connectors are available which use conductors fabricated from the same dissimilar metals as the thermocouple wires of the cable. Use of these connectors can maintain accuracy within the class 1 tolerance given above.
Known methods of providing disconnectable connections in thermocouple circuits include:—                Plugs and sockets made with screw-fit terminal materials matched to the particular thermocouple (e.g. push-fit connectors with trade names “Marlin” and “Mini-Marlin” connectors). These can be used with flexible polymer insulated cables (e.g. PTFE or polyimide insulation), and also with less flexible, mineral insulated (MI or MIC) cables for use at higher temperatures.        Pairs of thermocouple material contacts within multi-way connector housings.        
Mineral insulated cables are commonly used for their high temperature resistance, but are generally more difficult to terminate than flexible polymer insulated cables. It is common practice to transition from mineral insulated cable to flexible polymer tails in order to more easily apply contacts for insertion into disconnectable connections and for crimp spade terminations.
However, the use of intermediate connections (even when the conductors are fabricated from the same dissimilar metals as the thermocouple wires) or joints within batch wire calibrated cable can still result in a loss of accuracy when there are temperature differences between joints.
Japanese patent application no. 2001141571 proposes a thermocouple connector with male-type and female-type terminals. U.S. Pat. No. 4,863,283 proposes an electrical connection for a thermocouple lance which includes a temperature sensor in the connector.